1. Field of the Invention
The invention relates to a circuit board and a circuit apparatus using the same. In particular, the invention relates to a circuit board having a substrate consisting primarily of metal as a core member, and a circuit apparatus using the same.
2. Description of the Related Art
While portable electronics equipment including cellular phones, PDAs, DVCs, and DSCs are rapidly becoming more functionally sophisticated, miniaturization and weight savings have become essential in order for these products to be accepted in the market. To achieve this, more highly integrated system LSIs have become sought after. In the meantime, enhanced usability and convenience are also desirable features of such electronics equipment, and increased functional and performance sophistication have therefore been required of the LSIs intended for use in such equipment. Consequently, while the highly integrated LSI chips have facilitated the implementation of a greater numbers of I/Os, miniaturization of the packages themselves has now become imperative. In order to satisfy both of these requirements, there has been a strong demand to develop a semiconductor package suited to packaging semiconductor parts on a board at a higher density. To meet this demand, various types of packaging technologies called CSP (Chip Size Package) have been developed.
Among the known examples of such packages is a BGA (Ball Grid Array). With the BGA, semiconductor chips are mounted on a packaging substrate and molded with resin. Then, solder balls are formed over an area on the other side of the substrate as external terminals. Since the BGA realizes a planar mounting area, it is relatively easy to miniaturize the package. Therefore, the circuit boards need not be designed in narrower pitches, and do not require high-precision mounting technologies. The BGA can thus be used to reduce the total packaging cost, even if the package itself is of a relatively high cost.
FIG. 9 is a diagram showing the general configuration of a typical BGA. The BGA 100 has a structure where an LSI chip 102 is mounted on a glass epoxy substrate 106 via an adhesive layer 108. The LSI chip 102 is molded with sealing resin 110.
The LSI chip 102 and the glass epoxy substrate 106 are electrically connected with metal wires 104. An array of solder balls 112 are formed on the back side of the glass epoxy substrate 106. Through these solder balls 112, the BGA 100 is mounted on a printed wiring board.
In recent years, semiconductor packages (circuit apparatuses) to be incorporated in electronic equipment and the like have required greater miniaturization, higher densities, and increased functionality. The circuit apparatuses have thus grown in heat generation density per unit volume. Such increases in the heat generation density can cause adverse effects on the performance and reliability of the circuit apparatuses, thereby causing significant problems. For this reason, metal substrates and the like having high radiation performance have been used as circuit boards for constituting the circuit apparatuses, instead of the glass epoxy substrate 106. Furthermore, in order to conduct heat to the metal substrates efficiently, resins that contain alumina and other fillers having high thermal conductivities are used to make the insulating layers that constitute the circuit apparatuses. The heat radiation capabilities of the entire circuit apparatuses are therefore enhanced by these means.
FIG. 10 is a sectional view schematically showing the structure of a conventional circuit apparatus disclosed in Japanese Patent Laid-Open Publication No. 2002-335057. With reference to FIG. 10, the circuit board 210 for constituting a conventional circuit apparatus 200 includes a metal substrate 201 as a core member thereinside. Wiring pattern layers 203 and 205 are formed on both sides of the metal substrate 201 via resin insulating layers 202 and 204. For electrical connection between the layers, through apertures called through holes 206 are formed in the thickness direction. The inner surfaces of the through holes are plated with copper or the like, thereby forming an electrical conduction layer 207 for layer-to-layer conduction. In addition, a semiconductor chip 220 is directly connected to one side of the circuit board 210 via solder balls 221.
Resins containing no filler have thermal conductivities of approximately 2 W/(m·K). Filler-containing resins typically have thermal conductivities of approximately 10 W/(m·K), depending on such factors as the thermal conductivity of the filler itself, and the content of the filler. Consider the instance where that a filler-containing resin is used as the internal insulating layers of the metal substrate. The resin insulating layers 202 and 204 are then formed with the metal substrate 201 therebetween, or on the top and bottom surfaces of the same, by thermocompression under vacuum or reduced pressure. In this instance, the resin insulating layers 202 and 204 flow into and fill the through holes 206 from the top and bottom surfaces, respectively.
FIG. 11 is an enlarged sectional view of the circuit apparatus of FIG. 10, showing the vicinity of a through hole in the circuit board. It should be appreciated that the broken line in FIG. 11 indicates the center position (CA) of the metal substrate in the thickness direction.
Since the resin insulating layers 202 and 204 flow into and fill the interior of the through hole 206 from the top and bottom surfaces of the metal substrate 201 at the same time, the joining surface between the resin insulating layers 202 and 204 generally coincides with the center position (CA) of the metal substrate 201 in the thickness direction. While these resin insulating layers are flowing, their resin component and accompanying filler move at different rates of mobility (the mobility of the filler is lower than that of the resin component). The resin inside the through hole 206 thus has areas 202a and 204a where the content of the filler (filler concentration) is lower. Since these areas 202a and 204a have filler concentrations lower than that of the resin insulating layers lying outside the through hole 206, the surrounding areas including the through hole 206 are low in thermal conductivity and low in heat radiation capability.